Method and apparatus for reduction of the pressure in a liquid mixture

ABSTRACT

In connection with reduction of the pressure of newly produced oil, i.e. a mixture of hydrocarbon compounds, this conventionally is performed stepwise, a momentary pressure drop occurring at each stage. 
     According to the invention it is suggested that oil is caused to flow radially inward in a chamber (12), which is formed by a rotating vessel (1) and which is only partly filled with rotating oil, whereby the pressure drop in the oil can occur relatively slowly. A free liquid surface is maintained in the chamber (12), and a certain gas pressure is maintained in the liquid free part thereof, while evaporated hydrocarbon compounds are removed through a central gas outlet (18). By means of a special inlet device (14; 3a, 40) it is prevented that the pressure in the oil, when it is on its way through the channel (15; 41) radially outward in the vessel (1) to the level, from which it is to flow radially inward, increases in the way that would happen if the oil would be allowed to flow freely in the channel and simultaneously be completely entrained in the rotation of the vessel.

The present invention relates to a process for reduction of the pressurein a liquid mixture of hydrocarbon compounds. Such a process is to befound in connection with oil production, when oil flows up of a drillhole with an overpressure which is often very large, for instance in theorder of 75-100 bar. The composition of the oil may vary from one oilsource to another. For instance the following relatively easily volatilehydrocarbon compounds may be present in the oil: methane, ethane,propane, buthane, isobuthane, penthane, isopentane and hexane.

If the pressure in newly produced oil is reduced momentarily tosubstantially atmospheric pressure, a large part of the easily volatilehydrocarbon compounds therein will be evaporated. For several reasonsthis is not desirable. It has proved, however, that the amount ofhydrocarbon compounds being evaporated in connection with reduction ofthe oil pressure may be reduced substantially, if the pressure reductionis performed stepwise. Thus, in practice a method is used in which thepressure reduction is performed in 3-5 steps. Between the pressurereductions, which are performed momentarily in that oil is passedthrough throttles, the oil is caused to flow through a container inwhich it exposes a free liquid surface towards a space which containsevaporated hydrocarbon compounds at a thoroughly determined pressure.The evaporated hydrocarbon compounds are removed continuously from therespective containers without the pressures therein being reduced belowthe predetermined pressures.

It is considered that if the pressure reduction of the oil could beperformed in the described manner in even more and smaller steps, evenmore oil would remain in a liquid form. The additional cost forequipment for reducing the oil pressure in more than 3-5 steps has beenconsidered higher than the profit possible to obtain, however.

The object of the present invention is to provide a new method and anapparatus for reducing the pressure in newly produced oil, i.e. in aliquid mixture of hydrocarbon compounds, enabling that a larger part ofthe oil can be maintained in a liquid form to acceptable costs.

According to the invention this is possible in a way such, that theliquid mixture is supplied into a chamber, formed by a rotating vessel,at a first radial level in the chamber, the liquid mixture beingsupplied to the vessel at a predetermined pressure in an area close tothe rotational axis of the vessel and being conducted further through achannel radially outward in the vessel to said first radial level in away such that the pressure of the liquid mixture will not besubstantially lower than said pressure, and the pressure of the liquidmixture, when the latter reaches said first radial level, issubstantially lower than it would be if the liquid mixture in thechannel would be allowed to flow freely therein and simultaneously becompletely entrained in the rotation of the vessel; that the liquidmixture being supplied to the chamber is brought into rotation thereinby means of the vessel; that a free liquid surface is maintained of therotating liquid mixture in the chamber at a second level radially insideof said first level; that a gas pressure lower than said predeterminedpressure is maintained in the liquid free part of the chamber; thatliquid is removed from the chamber at a level radially inside said firstlevel; and that gas is removed from the liquid free part of the chamber.

By this invention it can be avoided that the liquid mixture is subjectedto momentary pressure drops. Instead, the pressure in the liquid mixturemay be reduced relatively slowly, while a free liquid surface of theliquid mixture is maintained in contact with evaporated hydrocarboncompounds at a desired pressure and at a desired temperature. Within thescope of the invention several chambers of the described kind may becoupled in series, gradually lower gas pressures being maintained in thechambers.

According to the invention the pressure in the liquid mixture should bereduced by bringing the liquid mixture to flow radially inward in arotating vessel, i.e. by subjecting it to a gradually decreasingcentrifugal force. Further, according to the invention, the liquidmixture on its way to the treatment chamber in the vessel should beprevented from being influenced by the rotation of the vessel in thesame way as the liquid mixture which is already present within thechamber. In other words it has to be prevented that the pressure in theliquid mixture, when it is on its way radially outward in the vessel,increases in the way that would happen if the liquid mixture would beallowed to flow freely and simultaneously be entirely entrained in therotation of the vessel. In order to avoid that the liquid mixture isinfluenced in this way by centrifugal forces on its way into thetreatment chamber of the vessel, i.e. before it is introduced into thechamber at said first radial level, the liquid mixture may be suppliedin different ways.

According to one example of performing the method according to theinvention the liquid mixture may be conducted into the chamber through achannel in a stationary member, which extends centrally into therotating vessel and further radially outward therein via the liquid freepart of the chamber and into the liquid body rotating in the chamber.

According to another example of performing the method according to theinvention the liquid mixture may be conducted into the treatment chamberthrough means rotatable together with the vessel and forming asubstantially annular channel extending from an area near the rotationalaxis of the vessel to said first radial level in the chamber, saidchannel being substantially free of entrainment members, so that theliquid mixture flowing through the channel during the rotation of thevessel is only insignificantly entrained in the rotation of the vessel.

The invention is described more closely in the following with referenceto the accompanying drawing, in which FIG. 1 shows a rotatable vesselbeing a part of an apparatus according to the invention, FIG. 2 shows amodified inlet device in a rotatable vessel according to FIG. 1, andFIG. 3 schematically shows the whole apparatus according to theinvention.

In FIG. 1 there is shown a vessel 1 of a rotational symmetrical shapecomprising a surrounding wall 2 and two end walls 3 and 4. By means ofbearings 5 and 6 the vessel is rotatably supported by two stationarysupport members 7 and 8. Via a driving device 9 the vessel is rotatablycoupled to a driving motor 10 (FIG. 3).

By an annular partition 11 within the vessel 1 the interior thereof isdivided in two chambers 12 and 13. In the chamber 12 there is placed anannular stationary inlet member 14, which has several separate inletchannels 15 extending from the rotational axis of the vessel to thevicinity of the surrounding wall 2 of the vessel, where they open intothe chamber 12. The inlet member 14 at its centre has a cylindricalpart, extending out of the vessel 1 and having a central inlet channel16, with which all of the channels 15 are communicating.

Within the chamber 12 the vessel 1 supports centrally a stack of conicaldiscs 17 arranged coaxially with the vessel at some axial distance fromeach other. The discs 17 surround a free space 18, which through thedisc interspaces communicates with the chamber 12 and through a firstgas outlet channel 19 communicates with a first gas outlet 20 in thestationary supporting member 8.

Within the chamber 13 the vessel 1 supports centrally a further stack ofconical discs 21 similar to the discs 17 in the chamber 12. The discs 21surround a free space 22, which through the disc interspacescommunicates with the chamber 13 and through a second gas outlet channel23 communicates with a second gas outlet 24 in the supporting member 8.

The chambers 12 and 13 communicate with each other via an annular gap25, which is formed between the surrounding wall 2 of the vessel and thepartition 11. The slot 25 is situated at a level in the vessel 1radially inside of the openings of the inlet channels 15 in the chamber12.

On its inside in the chamber 12 the surrounding wall 2 supports anannular threshold member 26. The radially innermost portion of thisthreshold member is situated close to and radially somewhat inside ofthe connecting slot 25 between the chambers 12 and 13. Between thethreshold member 26 and the partition 11 there is formed an annularpassage 27.

In the chamber 13 the surrounding wall 2 of the vessel supports afurther annular threshold member 28. The radially innermost portion ofthis threshold member is situated at a level radially inside of the slot25. Between the threshold member 28 and the end wall 4 of the vesselthere is formed an annular groove 29, which is open radially inward. Outinto the groove 29 there is extending a stationary annular outlet member30 arranged coaxially with the vessel 1. The outlet member 30, which isconnected with the supporting member 8, has one or more outlet channels31. These extend from one or more openings in the outlet member 30 atthe area of the groove 29 radially inward and further out of the vessel1 to an outlet chamber 32 in the supporting member 8. An outlet conduit33 is connected to the supporting member 8.

In the chamber 12 the member supporting the conical discs 17 issupported radially by the surrounding wall 2 of the vessel throughseveral radially and axially extending wings 34, which are evenlydistributed around the rotational axis of the vessel. These wings alsoform entrainment members in the chamber 12, so that the liquid mixturesupplied to the chamber is given the rotational movement of the vessel.Similar wings 35 are present in the chamber 13.

For sealing of the chambers 12 and 13 from the surrounding of the vesselthere are sealing members 36 between the supporting member 7 and the endwall 3, sealing members 37 between the supporting member 8 and the endwall 4 and sealing members 38 between the supporting member 8 and acentral pipe 39 forming the above said gas outlet channel 19 from thechamber 12.

In FIG. 2 there is shown an alternative embodiment of the inlet deviceof the vessel 1 for liquid mixture. Details in FIG. 2 which haveidentical counterparts in FIG. 1 have the same reference numerals as inFIG. 1. Details in FIG. 2, which have counterparts in FIG. 1 but whichas to their form are somewhat modified, have the same reference numeralsas in FIG. 1 with the addition of letter a.

The inlet device in FIG. 2 comprises a conical partition 40 whichthrough wings 34a is connected with the surrounding wall 2 of the vesseland, thereby, is arranged to rotate with the vessel 1. Between the endwall 3a of the vessel and the partition 40 there is formed an annularchannel 41. The channel 41 communicates at the vessel axis with acentral channel 42 formed by a cylindrical portion of the end wall 3aand in turn communicating with an inlet channel 16a, which is defined bythe supporting member 7a.

In FIG. 3 an apparatus according to the invention is shownschematically. Thus the rotatable vessel 1 is shown with its drivingdevice 9, 10, its inlet channel 16 for liquid mixture, the gas outlets20 and 24 and the outlet conduit 33 for liquid mixture.

The gas outlet 20 contains a control device 43 of a conventional kindfor maintaining of a certain predetermined gas overpressure in thechamber 12 of the vessel. The gas outlet 24 contains a similar controldevice 44 for maintaining of a predetermined lower gas overpressure inthe other chamber 13 of the vessel.

The liquid outlet conduit 33 contains a counter pressure device 45, bymeans of which a desired counter pressure may be set for the liquid flowout through the outlet member 30.

The apparatus according to the invention operates in the followingmanner with an inlet device in the vessel 1 of the kind shown in FIG. 1.It is assumed that the vessel 1 is rotating and that the inlet channel16 is connected to an overpressure source for a liquid mixture ofhydrocarbon compounds.

With the pressure it has in the inlet channel 16 the liquid mixture isconducted further through the radial channels 15 and out into thechamber 12 near the surrounding wall 2 of the vessel. In this part ofthe chamber 12 the liquid mixture is given the same rotational speed asthe vessel 1 by means of the wings 34. Between the wings 34 the liquidmixture flows radially inward and axially in the chamber 12, passes overthe threshold member 26 and leaves the chamber 12 via the slot 27. Whenthe liquid mixture enters the chamber 13 it flows further on between thewings 35 radially inward and axially towards the outlet member 30. Inthe chamber 13 the threshold member 28 forms an overflow outlet for theliquid mixture, before it enters the groove 29. Through the channels 31in the stationary member 30 the liquid mixture leaves the chamber 13. Bymeans of the counter pressure device 45 (FIG. 3) the counter pressure inthe outlet concuit 33 is set such that all the liquid mixture flowingover into the groove 29 will be discharged out through the outlet member30 without causing the free liquid surface in the groove 29 to moveradially inside of the threshold member 28.

Hereby, the threshold member 28 will maintain a free liquid surface inthe chamber 13 at the same level as its radially innermost portion. Thepressure from a liquid column, which will thereby be present within thechamber 13 radially inside of the connecting slot 25 between thechambers 12 and 13, as well as the overpressure from the gas (evaporatedhydrocarbons) present in the chamber 13 and acting on the free liquidsurfaces therein, will determine the position of the free liquid surfaceformed in the chamber 12. The counter pressure in the slot 25 from thechamber 12 is constituted by the overpressure of the gas (evaporatedhydrocarbons), which is present in the liquid free part of the chamber12, and the pressure of the liquid column formed in the chamber 12between the free liquid surfaces therein and the slot 25.

The liquid mixture having been supplied to the vessel 1 via the inletchannel 16 will maintain substantially its pressure while it is flowingthrough the channels 15 and into the chamber 12. The pressure of therotating liquid in the chamber 12 near the openings of the channels 15therein will thus be adjusted such that it substantially corresponds tothe pressure of the liquid mixture in the inlet channel 16. If saidpressure of the rotating liquid is too high, the liquid mixture can notflow into the chamber 12. If the pressure is too low, a pressure dropmay arise in the liquid mixture on its way through the channels 15,resulting in an undesired evaporation of hydrocarbons in these channels.

After the liquid mixture has entered the chamber 12 it flows radiallyinward and axially towards the passage 27. During this course the liquidmixture is subjected to a gradual pressure drop, part of its contentbeing evaporated and leaving the vessel through the central part of thechamber 12. Possibly entrained liquid drops are separated in thepassages between the conical discs 17 and are thrown back to the freeliquid surface in the chamber 12.

Still entrained in rotation by the wings 34, which extend even throughthe passage 27, the liquid mixture flows further on through the passage27 and the slot 25 into the radially outermost part of the chamber 13.In the slot 27 the pressure is somewhat increasing in the liquidmixture, but in the chamber 13 it is decreasing again, when the liquidmixture flows radially inward and axially toward the outlet member 30. Afurther part of the liquid mixture is then evaporated in the chamber 13and is discharged via its liquid free part from the vessel 1.

After its passage of the threshold member 28 the liquid mixture will goon rotating in the groove 29 entrained by the wings 35 which extend intoa part of the groove. The pressure in the rotating liquid mixture aswell as the gas pressure in the chamber 13 are accomplishing a liquidtransport radially inward in the channels 31 of the staionary outletmember 30 and further out through the outlet member 32 and the outletconduit 33. Possibly the outlet member 30 may be formed as a paringmember in the groove 29, so that even the movement of the liquid mixturein the peripheral direction of the groove may be used for the liquidtransport through the outlet channels 31.

The inlet device according to FIG. 2 operates in the following manner.

The liquid mixture entering through the channel 16a with a predeterminedoverpressure flows further on through the annular channel 41 into thechamber 12a. In the channel 41 thin border layers of the liquid mixturewill be caused to rotate by the end wall 3a of the vessel and thepartition 40, whereas the main part of the liquid will not be given anysubstantial rotational movement. It is thus possible even by means ofthis inlet device to maintain substantially the same overpressure in theliquid mixture during its flow from the central inlet channel 16a intothe chamber 12a, where the liquid mixture is rotating with the vessel 1.

For accomplishing the best possible flow conditions, when the liquidmixture flows into the chamber 12 or 12a, respectively, differentarrangement can be used. For instance, at an inlet device according toFIG. 1 the openings of the inlet channels 16 into the chamber 12 may beturned in the rotational direction of the vessel 1. At an inlet deviceaccording to FIG. 2 there can be arranged in the radially outermost partof the channel 41 a number of conical, annular discs--coaxial with thevessel 1 and rotatable therewith--which form between themselvesrelatively narrow annular passages for the incoming liquid mixture. Thefunction of such discs is that they would successively increase thecontact surface between the liquid mixture and rotating surfaces in thevessel for avoiding too heavy chocks at the acceleration of the liquidmixture to full rotational speed at the transfer between the channel 41and the chamber 12a.

A rotatable vessel has been described above containing two chambers 12and 13, in which there are maintained different gas pressures at therespective free liquid surfaces. This should be seen as a furtherdevelopement of a basic embodiment of the invention, in which the vesselcontains only one chamber and one single outlet for evaporatedhydrocarbon compounds. Since the chambers 12 and 13 communicate throughthe passage 27 and the slot 25, the chambers may be considered as oneand the same chamber, however, concerning the treated liquid. Onlyconcerning the evaporated hydrocarbon compounds the chambers 12 and 13need be considered as separate.

Within the scope of the invention it is of course possible in this wayto divide the interior of the vessel in even more chambers, graduallydecreasing gas pressures being maintained therein.

It is also possible according to the invention to connect two or morevessels of the described kind in series. i.e. to connect the liquidoutlet 33 of one vessel to the liquid inlet 16 of a subsequent vessel.For instance in this way the pressure in a liquid mixture may be reducedfrom 60 to 15 bar in a first vessel rotating with a certain speed, andfrom 15 to 5 bar in a second vessel having the same size but rotatingwith a lower speed.

A pressure reduction vessel of the kind here described may also be usedas a centrifugal separator for separating from the supplied liquidmixture of hydrocarbon compounds for instance water and/or solids. Thevessel in a case like that may be equipped with conical separationdiscs, like conventional centrifugal separators, and with members forcontinous or intermittent discharge of substances thus separated.

As defined by the accompanying claims it should be prevented accordingto the invention that the pressure of the liquid mixture, that isflowing radially outward in the vessel through a channel to thetreatment chamber inlet at said first radial level, increases in the waywhich would happen it this liquid would be allowed to flow freely in thechannel and simultaneously be completely entrained in the rotation ofthe vessel.

According to a special embodiment of the invention, which is not shownin the drawing, this is obtainable in the following alternative way.

At an arrangement according to FIG. 2 several frusto-conical discs maybe arranged coaxially in the annular channel 41, connected with thevessel 1 for rotation therewith. The distance between these discs may bemade so small that the liquid mixture, which is introduced centrally inthe disc interspaces, as completely entrained by the discs but isprevented from flowing freely in the interspaces, whereby it will besubjected to a substantial dynamic pressure drop on its way towards thesurrounding wall 2 of the vessel. Such a dynamic pressure drop may alsobe accomplished by members formed in a different way.

I claim:
 1. A method for reducing pressure in a liquid mixture ofhydrocarbons which comprises supplying the liquid mixture at apredetermined pressure to a rotating vessel at a point near itsrotational axis, conducting the mixture radially outwardly through achannel to an inlet of a chamber formed in the vessel, said inlet beingsituated at a first radial level, the mixture being conducted throughsaid channel in a manner to maintain the mixture at a pressure notsubstantially lower than said predetermined pressure but substantiallylower than it would have been at said first radial level if the mixturehad been allowed to flow freely in the channel and been simultaneouslyentrained in the rotation of the vessel, bringing the liquid mixture inthe chamber into rotation with the vessel by means of entrainmentmembers connected with the vessel, maintaining a free liquid surface insaid chamber, at a second radial level inwardly of said first level,maintaining a part of said chamber liquid-free radially inwardly of saidfree liquid surface to receive gas evaporated from said mixture,maintaining a gas pressure lower than said predetermined pressure in theliquid-free part of said chamber, removing liquid from the chamber at aradial level inwardly of said first level, and removing liquid and gasfrom the vessel, gas being removed from the liquid-free part of thechamber.
 2. Method according to claim 1, wherein the liquid mixture issupplied at the first radial level in the chamber by means of astationary member extending through the liquid-free part of the chamber.3. Method according to claim 1, wherein the liquid mixture is conductedfrom an area near the rotational axis of the vessel to the first radiallevel through an annular channel formed by members rotating with thevessel.
 4. Method according to any of the preceding claims, wherein thefree liquid surface in the chamber is maintained at said second level bymeans of an overflow outlet.
 5. Apparatus for reducing the pressure in aliquid mixture from a predetermined pressure comprising a rotatablevessel having a chamber, means for rotation of said vessel about anaxis, inlet means for supplying a liquid mixture into said chamber at afirst radial level and free from means for inducing substantial rotationof said liquid mixture, said inlet means having at least one channelextending from an area near the rotational axis of the vessel to saidfirst radial level and being arranged to prevent a pressure increase inthe liquid mixture in the channel such as would result from allowing theliquid mixture to flow freely in the channel and be simultaneouslycompletely entrained in the rotation of the vessel, members connectedwith the vessel and arranged for entraining liquid mixture in saidchamber in the rotation of the vessel, means for maintaining a freeliquid surface of the liquid mixture in the chamber at a second levelradially inside said first level thereby to provide a liquid-free gasreceiving space radially inwardly of said free liquid surface, means fordischarging liquid mixture from the chamber at a level radially insidesaid first level, means for discharging gas from the liquid-free part ofthe chamber, means for removing gas and liquid from the vessel, gasbeing removed from said gas receiving space and means for maintaining agas pressure in the liquid-free part of the chamber which is lower thansaid predetermined pressure.
 6. Apparatus according to claim 5, whereinthe inlet means comprises a stationary member which extends axially intothe vessel at the rotational axis thereof and radially outward withinthe vessel to said first level in the chamber.
 7. Apparatus according toclaim 5, wherein the inlet means comprises means rotatable with thevessel and forming a substantially annular channel extending from anarea near the rotational axis of the vessel to said first radial levelin the chamber, said channel being substantially free of entrainmentmembers, so that liquid mixture flowing through said channel during therotation of the vessel is only insignificantly entrained in therotation.
 8. Apparatus according to claim 5, and including means formingan overflow outlet from the chamber and constituting both part of thesaid liquid discharge means and part of said means for maintaining afree liquid surface in the chamber.
 9. Apparatus according to claim 5and wherein said vessel comprises two chambers, one chamber having aninlet for liquid mixture at the first radial level and outlet for liquidmixture at a level radially inside said first level and an outlet forgas from a part of said chamber which is free of liquid during rotationof the vessel, and the other chamber having an inlet for liquid mixture,an outlet for liquid mixture at a level radially inside the level of itssaid inlet and an outlet for gas from a part of the chamber which isfree of liquid during the rotation of the vessel, the outlet for theliquid mixture of said one chamber communicating with the inlet of saidother chamber, said apparatus further comprising a first device arrangedto maintain a predetermined first overpressure of the gas in theliquid-free part of the first chamber, and a second device arranged tomaintain a predetermined second lower overpressure of the gas in theliquid-free part of the other chamber, and means arranged to obtain afree liquid surface at a predetermined level in at least one of the twochambers.